Sea-level rise drives shoreline retreat in Hawaii

Example of chronic coastal erosion threatening a building on Maui. (Photo by Zoe Norcross-Nuu)

Sea-level rise has been isolated as a principal cause of coastal erosion in Hawaiʻi. Differing rates of relative sea-level rise on the islands of Oʻahu and Maui remain as the best explanation for the difference in island-wide shoreline trends (that is, beach erosion or accretion) after examining other influences on shoreline change including waves, sediment supply and littoral processes, and anthropogenic changes.

Knowing that sea-level rise is a primary cause of shoreline change on a regional scale allows managers and other coastal zone decision-makers to target sea-level rise impacts in their research programs and long-term planning. This study is confirmation that future sea-level rise is a major concern for decision-makers charged with managing beaches.

“It is common knowledge among coastal scientists that sea-level rise leads to shoreline recession,” said Brad Romine, coastal geologist with the Sea Grant College Program. “Shorelines find an equilibrium position that is a balance between sediment availability and rising ocean levels. On an individual beach with adequate sediment availability, beach processes may not reflect the impact of sea-level rise. With this research, we confirm the importance of sea-level rise as a primary driver of shoreline change on a regional to island-wide basis.”

Globally averaged sea-level rose at about 2 mm per year over the past century. Previous studies indicate that the rate of rise is now approximately 3 mm per year and may accelerate over coming decades. The results of the recent publication show that sea-level rise is an important factor in historical shoreline change in Hawaiʻi and will be increasingly important with projected sea-level rise acceleration in this century. “Improved understanding of the influence of sea-level rise on historical shoreline trends will aid in forecasting beach changes with increasing sea-level rise,” said Charles Fletcher, associate dean and professor of geology and geophysics at SOEST.

Results of island-wide historical trends indicate that Maui beaches are significantly more erosional than beaches on Oʻahu. On Maui, 78 percent of beaches eroded over the past century with an overall (island-wide) average shoreline change rate of 13 cm of erosion per year, while 52 percent of Oʻahu beaches eroded with an overall average shoreline change rate of 3 cm of erosion per year.

“The research being conducted by SOEST provides us with an opportunity to anticipate sea-level rise effects on coastal areas, including Hawaiʻi’s world famous beaches, coastal communities and infrastructure. We hope this information will inform long-range planning decisions and allow for the development of sea-level rise adaptation plans,” said Sam Lemmo, administrator, Department of Land and Natural Resources, Office of Conservation and Coastal Lands.